Transparent conductive windows through which optical energy must pass and which have good electrical conductivity have utility in a number of applications. These include resistance heated windows, electromagnetic interference (EMI) shielded windows, anti-static windows and transparent electrodes.
In U.S. Pat. Nos. 4,778,731, 4,939,043, and 5,173,443, transparent conductive windows, coatings and methods of manufacture are described. In these patents the conductive component was either a thin coating applied by vacuum deposition techniques or a doped layer of semiconductor fabricated, in situ, into the structural window substrate made of the same semiconductor, or at least a compatible semiconductor.
Windows fabricated utilizing a thin conductive coating applied by vacuum deposition require the use of substrate materials that are limited by that coating. In general, such thin conductive coatings are very heavily doped amorphous semiconductors. Such materials exhibit high infrared absorption due to free carrier absorption. To achieve high optical transmission, the layer conductivity must be maximized. This tradeoff of conductivity for transmission usually limits this approach to window substrate materials that are semiconductors having either single crystal or large-grain polycrystalline structures. Thus the substrate sizes available restricts the final window dimensions. Additionally, the doped semiconductor coating fabrication is an expensive procedure.
Windows fabricated using a semiconductor as both the structural component (substrate) and the conducting coating are limited by the thickness of the semiconductor needed for structural strength. While the preferred semiconductors, Si, Ge, and GaAs, are low absorbing materials, they do have absorption bands within the normal optical bands of interest for windows. When windows are constructed with thickness needed for structural strength the transmission loss due to this absorption can be substantial.
Window material which are highly transparent in the infrared, such as ZnSe, are polycrystalline materials having small grain sizes. These materials cannot be doped to achieve a low loss conducting coating. Growth of a conducting coating on them via heteroepitaxy results in low mobility and, therefore, optically lossy windows.
There is, therefore, a need for an improved transparent, electrically conductive window.